Birefringent optical fibers for sensing parameters such as pressure are known in the art. In a birefringent fiber, birefringence is caused in part by the geometrical asymmetry that occurs when the optical fiber deforms under strain. However, because optical fibers are made of glass, and are typically fragile and small, they are relatively difficult to deform, which limits the sensitivity of such sensors. Typical fiber sensors, such as those made from standard fiber optic communication cables, have outer diameters in the range of 125 microns with optical cores of 7 to 12 microns and therefore have relatively low clad-to-core ratios.
Side-holes have been incorporated into fiber as is disclosed, for example, in U.S. Pat. No. 6,208,776, entitled “Birefringent Fiber Grating Sensor and Detection System,” which is incorporated herein by reference. By incorporating side-holes into the cladding of the fiber, the fiber's mechanical compliancy is increased as well as its potential sensitivity and range when used as a sensor. Birefringent fibers have also in the prior art incorporated specific sensing elements such as Bragg gratings for measuring desired parameters including pressure and temperature, as is disclosed in U.S. Pat. No. 6,304,686, entitled “Methods and Apparatus for Measuring Differential Pressure with Fiber Optic Sensor Systems,” which is incorporated herein by reference.
However, fiber optic based birefringent sensors are limited by their physical characteristics and manufacturing difficulties. For example, fiber sensors may not be subject to large pressures parallel to the axis of the fibers because the fibers may buckle. Additionally, fibers are small and delicate, and require special care during handling and manufacturing. Additionally, the protective buffer coating typically formed on standard optic cable has to be contented during manufacturing as one skilled in the art will understand, which adds manufacturing complexity and hence extra time and cost. Manufacturing yields for standard fiber-based sensors containing gratings can be lower than 10%, which is clearly not optimal. Formation of the side-holes in the relatively small cladding of the fiber can also be difficult to accomplish.
A waveguide that has been used to counteract some of the difficulties associated with optical “fibers” is a waveguide with a diameter ranging from about 0.3 mm to 4 mm, referred to as a “cane.” Cane waveguides have a core and a cladding just as do standard fibers. In fact, the core of a single mode cane is generally the same diameter as the core of a single mode standard fiber, typically 7 to 12 microns. However, cane is thicker and sturdier than fiber because of the substantial amount of cladding. While a standard fiber has a diameter of 125 microns, cane ranges from 0.3 mm to about 4 mm, the great bulk of which constitutes cladding. The cane's relatively thick cladding provides significant mechanical benefits over fiber. Furthermore, a cane does not require a protective buffer layer, and thus eliminates manufacturing complexity.
The art would benefit from ways to improve the performance of pressure and temperature sensing in a side-hole fiber by utilizing the structure of a cane. Such an improvement is disclosed herein, specifically a cane-based side-hole sensor which has improved sensitivity, is easier to manufacture, handle, and package, is more resilient, and which otherwise substantially eliminates the shortcomings of fiber-based side-hole sensors. In particular, the art of oil/gas production would especially benefit from improved pressure sensors utilizing sturdier cane-based structures which are suitable for deployment in harsh environments such as oil/gas wells.